SU‐E‐J‐157: Investigation of Gated Cone‐Beam CT to Reduce Respiratory Motion Blurring

Abstract

Purpose: Respiration‐correlated cone‐beam CT (RC‐CBCT) using slow continuous gantry rotation is commonly used to reduce respiratory motion blurring but image quality is limited by uneven projection spacing. This study investigates the efficacy of a novel gated CBCT technique (gantry motion and imaging only within gate) in reducing motion blur with improved image quality using a new type of computer‐controlled linac (Varian TrueBeam).Methods: For gated CBCT, the linac is programmed to gate gantry rotation and kV image acquisition with approximately 25% duty cycle around end expiration. The two‐part study examines images in phantom and prospectively in patients with thoracic and abdominal tumors. A motion phantom is programmed to follow a patient respiratory trace (1.4cm mean excursion) and CBCT scans are performed in standard “free‐breathing”, RC‐CBCT, and gated CBCT modes. A standard CBCT of the stationary phantom serves as ground truth. Evaluation of phantom image quality compares contrast‐to‐noise ratios (CNR) of spheres embedded in acrylic, delineated on the static image and aligned to the other images. In patient studies, image fidelity in standard free‐breathing and gated CBCT is compared relative to a respiration‐correlated CT (RCCT) on the same day, by computing a cross‐correlation metric (CCM) in an ROI around the internal target volume following rigid registration.Results: CNR in motion phantom for gated CBCT is nearly that for static phantom (ratio gated/static 0.95) and larger than for RC‐CBCT (ratio 1.64) and standard CBCT (1.22). CCM of the tumor ROI in gated CBCT is larger than standard CBCT in all three thoracic cases thus far (ratio gated/standard mean 1.04 range 1.03–1.06), indicating higher congruence with the ground‐truth RCCT Conclusions: Gated CBCT visibly reduces image blurring caused by respiratory motion. By exploiting recent computer‐controlled linac technology, complete projection sets can be acquired within a gate, yielding improved reconstruction image quality relative to RC‐CBCT.